Understand transformers the easy way, with plain English intuition, a clear labelled diagram, the turns ratio, ideal power conservation, worked examples and an auto marked practice test. VCE Physics Units 3 and 4.
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Plug in a phone charger and something quietly clever happens inside it. The power point gives you 230 V, but your phone wants only about 5 V. A transformer is the device that changes one AC voltage into another, and it does it with nothing more than two coils of wire sharing an iron core. No moving parts, just magnetism doing the work.
Two coils, one core
A transformer is two coils of wire wound onto the same loop of iron. You send an alternating current into the first coil and you draw an output from the second.
The primary coil is the input. It is connected to the AC supply, with voltage V1 and N1 turns.
The secondary coil is the output. It feeds the load, with voltage V2 and N2 turns.
The alternating current in the primary makes a changing magnetic flux in the iron core. The core guides that flux across to the secondary, where the changing flux induces an EMF. That is the whole machine.
The blue side on the left is the primary, driven by the AC source at V1. The red side on the right is the secondary, delivering V2 to the load. The grey square in the middle is the iron core that carries the flux between them.
The turns ratio
Here is the rule that makes transformers easy. The ratio of the voltages is the same as the ratio of the number of turns on each coil. More turns, more voltage.
V2V1=N2N1
So if the secondary has more turns than the primary, the output voltage is higher. That is a step-up transformer. If the secondary has fewer turns, the output voltage is lower, and that is a step-down transformer. The phone charger is step-down, the power-line transformer at the station is step-up.
You cannot get power for free
It is tempting to think a step-up transformer gives you something for nothing, because the voltage goes up. It does not. In an ideal transformer no power is wasted, so the power going in equals the power coming out.
V1I1=V2I2
This is the catch. When a step-up transformer raises the voltage, the current must drop by the same factor, so the power stays the same. A step-down transformer does the reverse: the voltage falls and the current rises.
See it for yourself
Open the Transformer panel, then change the number of turns on each coil. Watch the voltage and the bulb brightness in the second coil follow the turns ratio, and notice it only works while the current keeps changing.
A transformer cannot run on a steady DC supply, and the reason is the heart of how it works. An EMF is induced in the secondary only when the flux through it is changing. A steady DC current makes a steady, unchanging flux, so no EMF is induced and the secondary stays dead. Alternating current is always rising and falling, so it keeps the flux changing the whole time, which is exactly what the secondary needs.
How to actually solve one
The method is a short, repeatable recipe.
To find a voltage, use the turns ratio V2V1=N2N1 and rearrange for the unknown.
Check whether it is step-up or step-down by comparing the turns. More secondary turns means voltage up.
To find a current, use the ideal transformer rule V1I1=V2I2 and rearrange for the unknown.
Sanity check: if the voltage went up, the current should have come down, and the other way round.
Lock it in with active recall
Cover the answer and say each one out loud before you flip. Rate yourself honestly — the cards you find hard come back sooner, the ones you know are spaced further out.
Active recall
Answer from memory first, then flip. Rate yourself and each card returns on a spaced schedule (1 → 3 → 7 → 16 days).
Write the turns ratio and what it links.
V2V1=N2N1 — the ratio of voltages equals the ratio of turns. More turns, more voltage.
What makes a transformer step-up versus step-down?
More turns on the secondary than the primary steps the voltage up; fewer turns steps it down.
For an ideal transformer, how do input and output power relate?
V1I1=V2I2 — power in equals power out. Raise the voltage and the current falls by the same factor.
Does a step-up transformer give you extra power for free?
No. The current drops as the voltage rises, so the power stays the same. A real transformer even loses a little to heat.
Why does a transformer only work on AC, not steady DC?
It needs a changing flux to induce an EMF in the secondary. AC keeps the flux changing; steady DC gives a constant flux, so nothing is induced.
What does the iron core do?
It carries the changing flux from the primary across to the secondary, so almost all the primary’s changing field is felt by the secondary.
Recall · Transmission of Power
How is the line power loss related to the current?
Ploss=I2R — the loss depends on the square of the current, which is why transformers step the voltage up (current down) for transmission.
Recall · Generators and AC
What is RMS voltage for a sine wave, in terms of the peak?
Vrms=2Vpeak — the DC-equivalent voltage for heating.
See the recipe in action in the Worked Examples tab, then test yourself in Try It.
Worked examples
Worked Example 1Finding the secondary voltage
A transformer has 200 turns on the primary and 50 turns on the secondary. The primary voltage is 240 V. Find the secondary voltage.
1
The turns ratio links the voltages to the number of turns on each coil. Rearrange it to make the secondary voltage the subject.
V2V1=N2N1⟹V2=V1×N1N2
2
Put in the numbers. Fewer turns on the secondary than the primary, so expect the voltage to drop.
V2=240×20050=240×41=60 V
3
The secondary has fewer turns, so the voltage came down. This is a step-down transformer.
V2=60 V (step-down)
Answer
V2=V1×N1N2=240×20050=60 V
Worked Example 2Finding the secondary current
An ideal step-up transformer raises 100 V to 400 V. The primary current is 8.0 A. Find the secondary current.
1
An ideal transformer wastes no power, so the power going in equals the power coming out.
V1I1=V2I2
2
Rearrange to make the secondary current the subject, then substitute the values.
I2=V2V1I1=400100×8.0
3
Work it through. The voltage went up, so the current must come down to keep the power the same.
I2=400800=2.0 A
Answer
I2=V2V1I1=400100×8.0=2.0 A
Practice questions
Practice test
Try it yourself
6 questions, 10 marks
Choose your answers, then submit to see your score and the full worked solutions.
Multiple choice is marked for you, just like Exam 2 Section A.
Q1.A transformer will only work when it is supplied with:
1mark
Need a hint?
An induced EMF needs a changing flux. Ask which type of current keeps the flux changing.
Show worked solution
A transformer relies on a changing magnetic flux in the core to induce an EMF in the secondary coil. A steady DC current makes a steady flux, so nothing is induced. Only AC keeps the flux changing, so only AC makes a transformer work.
Q2.A step-up transformer increases the voltage. To do this, the secondary coil compared with the primary coil must have:
1mark
Need a hint?
Use V2V1=N2N1. More turns means more voltage.
Show worked solution
The turns ratio says V2V1=N2N1, so voltage and turns rise together. To step the voltage up, the secondary needs more turns than the primary. The current then drops to keep the power the same.
Q3.A transformer has 100 turns on the primary and 300 turns on the secondary. The primary voltage is 20 V. The secondary voltage is:
1mark
Need a hint?
Use V2=V1×N1N2.
Show worked solution
V2=V1×N1N2=20×100300=20×3=60 V. The secondary has more turns, so the voltage steps up. Dividing the turns the wrong way gives the trap answer of 6.7 V.
Q4.An ideal transformer steps 240 V down to 12 V. The current in the primary is 0.50 A. The current in the secondary is:
1mark
Need a hint?
For an ideal transformer V1I1=V2I2. Rearrange for I2.
Show worked solution
Power in equals power out, so V1I1=V2I2. Then I2=V2V1I1=12240×0.50=12120=10 A. The voltage stepped down, so the current stepped up.
Q5.An ideal transformer has 400 turns on the primary and 100 turns on the secondary. The primary is connected to a 200 V AC supply and draws a current of 1.5 A. Find the secondary voltage and the secondary current. Show your working.
3marks
Work this on paper. The worked solution appears once you submit.
Show worked solution
The turns ratio gives the secondary voltage:
V2=V1×N1N2=200×400100=50 V.
For an ideal transformer power in equals power out, so
I2=V2V1I1=50200×1.5=50300=6.0 A.
Q6.A wind turbine sends power along a transmission line to a distant shed. Two transformers are added: T1 at the turbine end and T2 at the shed end. State whether each of T1 and T2 should be a step-up or a step-down transformer, and justify each choice.
3marks
Work this on paper. The worked solution appears once you submit.
Show worked solution
T1 should be step-up: raising the voltage lowers the line current, which cuts the power wasted as heat in the wires (Ploss=I2R). T2 should be step-down: it brings the high transmission voltage back down to a safe, usable level for the equipment in the shed.
VCAA 2025 Physics Exam, Section B Q11
Frequently asked questions
Why do transformers only work with AC and not DC?
A transformer needs a changing magnetic flux to induce a voltage in the secondary coil. Alternating current is always changing, so it keeps the flux changing. A steady direct current gives a steady flux, so nothing is induced and the transformer does nothing. Switching a DC supply on or off does give a brief pulse in the secondary, because the flux changes for that instant, but a constant DC produces no ongoing output.
If a transformer steps the voltage up, do you get extra power for free?
No. In an ideal transformer the power in equals the power out, so when the voltage goes up the current comes down by the same factor. You never get more power out than you put in, and a real transformer loses a little to heat.
What does the iron core actually do?
The iron core carries the magnetic flux from the primary coil across to the secondary coil. It keeps almost all the changing flux flowing through both coils, so the changing field from the primary is felt strongly by the secondary.
